It is known that a nickel hydrogen secondary battery with a negative electrode containing a hydrogen storage alloy has a higher capacity and higher environmental safety than a nickel cadmium secondary battery, and has been therefore used for more and more applications including use in various apparatuses such as mobile electronic devices, electric power tools, and hybrid electric vehicles.
Due to the expanding applications, further increase in the capacity of a nickel hydrogen secondary battery is desired. One step for further increasing the capacity of a nickel hydrogen secondary battery is to improve the utilization factor of a positive electrode active material. More specifically, a conductive network of a cobalt compound with high conductivity is formed on the surface of nickel hydroxide particles as positive electrode active material. The formation of the conductive network improves the conductivity of the positive electrode active material, so that the utilization factor of the active material improves. Consequently, further increase in the capacity of a nickel hydrogen secondary battery can be achieved.
For use of a nickel hydrogen secondary battery, charging and discharging are repeated. The charge-discharge reaction associated with charging and discharging slightly reduces the valence of the cobalt compound forming the conductive network in the internal part of a nickel hydrogen secondary battery. As the number of repeating of the charge-discharge increases, the accumulated reduction in the valence of the cobalt compound results in gradual reduction in the conductivity. Consequently, the sufficient function of the conductive network cannot be achieved, so that the capacity of the battery gradually decreases. In addition, the cobalt compound forming the conductive network is partially reduced in the last stage of discharging, so that the elution phenomenon of the compound occurs. The reduction and elution of the cobalt compound causes the partial destruction of the conductive network, resulting in reduction in the conductivity of the positive electrode. Consequently, the charging acceptability is impaired and the capacity decreases due to the reduction in the utilization factor of the positive electrode active material.
As described above, a nickel hydrogen secondary battery having an increased capacity causes a problem of deterioration in cycle life properties due to reduction in the capacity associated with the increase in the number of the charge-discharge cycles. Accordingly, various trials have been made to improve the cycle life properties of a nickel hydrogen secondary battery (e.g. Patent Literature 1).
The nickel hydrogen secondary battery typically described in Patent Literature 1 has a positive electrode to which lithium hydroxide or lithium is added. The lithium hydroxide or lithium stabilizes the cobalt compound forming a conductive network and suppresses the reduction in the valence of the cobalt compound and the reduction/elution reaction associated with the charge-discharge cycle, so that the cycle life properties of the nickel hydrogen secondary battery can be improved.